1. Field of the Invention
The present invention relates to an aqueous sol of zirconia having lowered activity and a method of preparing the sol, and is more specifically directed to a stable aqueous sol of colloidal zirconia having a particle size of 20 to 1,500 nm and such a lowered surface activity that the colloidal zirconia, when adhered to a surface of a material, is removed from the surface by cleansing the material in water. The invention also relates to a method of preparing the sol.
2. Description of Related Art
U.S. Pat. No. 2,984,628, discloses a stable and acidic aqueous sol of a dense and ultimate particle of colloidal zirconia having a mean particle size (D) of 5 to 200 nm as observed by an electronic microscope, a specific surface area (A) of 5 to 400 in m.sup.2 /g as measured by the nitrogen gas adsorption method and a relationship A.times.D=1,000 to 2,000. The sol is prepared by a method comprising heating an aqueous solution of a salt of zirconyl at a temperature of 120.degree. to 300.degree. C. under pressure to cause the salt to hydrolyze.
Japanese laid-open patent publication No. Hei 2-167826 (1990) discloses a stable and alkaline aqueous sol of colloidal zirconia. The sol is prepared by a method comprising adding to an acidic aqueous sol of colloidal zirconia having a particle size of 50 nm an organic compound having a hydroxyl group which is soluble in water, and then adding to the resultant sol a basic compound so that a sol having a pH of 6 to 14 is formed.
The colloidal particles of zirconia in the sols as disclosed in these references and related prior art remain on a surface of a material such as, for example, a semiconductor material, with the colloidal particles adhered to the surface when the material is polished with the sols and thereafter rinsed with water, or remain on inner walls of containers with the colloidal particles adhered thereto when the sols have been contacted in the containers when the containers are empty and thereafter rinsed with water. Hence the colloidal particles of zirconia in the prior art are characterized as having a high surface activity or as having a highly active surface.